The electrical properties of superconducting materials, or superconductors, have led to the development of superconducting coils for storing large quantities of electrical energy in superconducting magnets. Typically, these are in the form of an annular solenoid installed such that it has a vertical axis. These solenoids are designed to have a substantial axial height and diameter, the diameter being on the order of about 500 to 1000 meters or more. Superconductive materials which are used in these devices, such as NbTi, are maintained at a temperature of approximately 1-4 degrees K (Kelvin) in order to exhibit superconducting properties. These temperatures can be maintained by surrounding the superconducting material within successive envelopes of cryogens having progressively higher boiling points.
Generally, the very low temperatures can be maintained by immersing the superconducting magnet in helium which can be made to have a boiling point lower than 4 K surrounded by, for example, another envelope of nitrogen which has a boiling point of about 77 K. Each of these cryogens can be maintained in the liquid state with a suitable refrigeration system.
During operation of such a superconducting magnet, the varying magnetic fields generated during operation can cause adjacent layers of the magnet structure, and particularly the layers of insulating material, to slide relative to one another thereby producing heat due to friction. This build up of heat must be prevented since sufficient heat generation can produce local hot spots within the magnet structure which can bring the conductor above its superconducting temperature. This can result in further heat generation which can cause the entire superconducting magnet coil to lose its superconducting properties and become normal. Such a sequence of events can cause catastrophic failure to the superconducting magnet due to heat build up from the extremely large electrical charge carried by the coil--on the order of about 50,000 Amps in the superconducting state, when the coil, in the normal state is capable of carrying only about 50-500 Amps.
A typical configuration for the superconducting magnet energy storage device includes several superconducting coils within a support structure, the support structures being secured together. The superconducting structures are joined at a lap joint, several structures joined to form the overall superconducting magnet. See FIG. 1. It is important to maintain a high contact pressure at these lap joints to prevent relative motion of one member with respect to another. Also, at certain times in the operation of the device, the joint is designed to conduct electricity from one member to the next; the amount of current that the joint can transmit is dependent upon the contact pressure. It is important that foreign materials be prevented from entering the superconducting magnet structure which could also cause loss of the superconducting properties. Thus, welding, brazing, and soldering of the lap joint are unacceptable due to the attendant problems of dirt and handling after the connection of the lap joint. Moreover, the joint must be able to withstand shear loads so that the joint itself is not damaged.
The superconducting magnet support structure is typically made of aluminum, and any fastener used must not damage this structure. Since this special care must be taken in the construction of the superconducting magnet, conventional hardware is not suitable for this purpose.